1. Field of the Invention
This invention relates to a defrost system for various types of refrigeration apparatus and more particularly, to an energy saving defrost system which avoids the introduction of external heat to defrost the condenser coils.
2. Description of the Prior Art
The accumulation of frost on evaporator coils has been a problem for many years. To overcome the problem, defrost systems have been developed and have been commercially successful.
The automatic defrost systems now in use employ heating elements to periodically melt the accumulated frost on the evaporators while the compressors are turned off. One such system is disclosed in U.S. Pat. No. 3,126,716 issued to DeWitte on Mar. 31, 1964. The DeWitte reference discloses a defrosting refrigerator wherein a fan circulates air throughout a freezer and refrigerator. A butterfly damper controls the air flow through the inlet of the refrigerator. A second damper controls the air flow to the inlet of the freezer space from the fan. The dampers are controlled by a thermostatically set bulb in the fluid motor. Three defrost heaters are turned on and the compressor is turned off when the temperature sensed near the evaporator is cooled below -4.5.degree. F. When the evaporator reaches a temperature of 37.degree. F., the heaters are turned off and the compressor started. The addition of heat to the system thus utilizes energy to melt ice on the coils and energy to cool the refrigerator to the extent of heat added by the heating coils. Thus, there is a two fold increase in energy requirements as a result of using heater coils for defrosting the evaporator coils.
Another device is disclosed in the U.S. Pat. No. 3,084,520 to Jacobs on Apr. 9, 1963. The Jacobs reference discloses an evaporater positioned above the freezer unit. When the compressor is running, the air passing through the evaporator coils is cooled sufficiently, a valve is opened to allow the cooled air to pass into the freezer unit. When the evaporator coils reach a temperature of 0.degree. F., the compressor is turned off and a heating bulb in the refrigerator unit is energized to promote heated convection currents to pass up behind the freezer unit and pass through the evaporator. The air inlet to the freezer unit is then closed so that the air passes through the evaporator coils to the refrigerator unit. When the air is sufficiently warm to melt the frost on the evaporator coils, the light bulb is shut off and the evaporator is restarted. The Jacobs system, like the DeWitte system, requires the addition of external heat to the system, thereby consuming energy in the defrost step and in the subsequent cooling step.
The U.S. Pat. No. 3,499,295 issued to Brennan on Mar. 10, 1970 discloses a freezer unit having two evaporator coils which alternately cool the air passing therethrough. One evaporator cools the freezer space, while the second evaporator is being defrosted. To this end, the warmed condenser fluid is passed through the defrosting evaporator coils prior to expansion and passing through the cooling evaporator coils. The system requires complex valving for the refrigeration fluid and does add some heat from the refrigeration fluid to the defrost coils. Thus, although the Brennan system avoids energy for the addition of heat inasmuch as the heat is already present in the refrigerator system, it still requires energy to cool the system to the extent that energy has been added to the system for defrosting the evaporator coils.